This is a continuation of services to other investigators to provide pregnancies of known gestational ages and to provide synchronized menstrual cycles in rhesus monkeys (Macaca mulatta). For timed breeding, the monkeys? cycles are controlled by the daily injection of progesterone (5 mg/day) for 10 days. Females are paired with males on days 14 to 21 after the last progesterone injection. Ovulation and conception occurs on day 16 to 18 after the progesterone injections. Success rate, based on viable fetus at 21 days gestation, remains at 60% to 70% for first cycle matings. Fetal development is monitored using ultrasonography. For synchronization of menstrual cycles monkeys are examined twice daily to detect menses. On the 16th days of the cycle daily progesterone injections begin. Injections are continued until all the monkeys to be synchronized have received a minimum of 10 injections. At that point all injections cease. Menses occurs on day 1 or 2 following the last progesterone inj ection. If the investigator wishes to treat the monkeys in the luteal stage of the cycle, progesterone injections can begin again on day 16 - 18 and the monkeys can be maintained in a luteal stage for the study. FUNDING Base Grant PUBLICATIONS None